1. Technical Field
The present invention relates to a power-supply device that supplies a voltage at a DC power supply to a load while boosting the voltage.
2. Related Art
Conventionally, various power-supply devices are well known in order to supply a DC voltage to various instruments and circuits, which are mounted on an automobile. For example, each of Japanese Unexamined Patent Publication Nos. 2005-112250, 2010-183755, 2011-162065, and 2005-160284 discloses a power-supply device including a DC-DC converter. The DC-DC converter includes a booster circuit, the booster circuit includes a switching element, a coil, and a capacitor, and the boosted DC voltage is output by switching the voltage at the DC power supply at high speed.
Some automobiles include what is called an idling stop function, in which the automobile automatically tentatively stops an engine when waiting at a stoplight and automatically restarts the engine during starting. In the automobile including the idling stop function, because a large current is passed through a starter motor during the engine restart, a battery voltage drops largely to generate such an abnormal state that the instrument or the circuit is reset. Therefore, it is necessary to boost the battery voltage in order to compensate the voltage drop.
In the power-supply devices disclosed in Japanese Unexamined Patent Publication Nos. 2005-112250, 2010-183755, and 2011-162065, the DC-DC converter is provided between the battery and the load, and a bypass relay constituting a bypass path is provided with respect to the DC-DC converter. The DC voltage is supplied from the battery to the load through the bypass relay during a normal run, and the boosted DC voltage is supplied from the battery to the load through the DC-DC converter during the engine restart. Therefore, the drop of the power supply voltage can be compensated during the engine restart to normally operate the instrument or the circuit, which is of the load.
The power-supply device disclosed in Japanese Unexamined Patent Publication No. 2005-160284 is mounted on an electric automobile, and the DC-DC converter is provided between the battery and an inverter in order to compensate the battery voltage drop due to a back electromotive force generated by the motor during the high-speed rotation of the motor. The bypass relay constituting the bypass path is provided with respect to the DC-DC converter. Whether the DC voltage at the battery is supplied to the load through the bypass relay or the DC-DC converter is switched based on an instruction from a feedback means.
FIG. 17 illustrates an example of the conventional power-supply device including the DC-DC converter. A power-supply device 300 is provided between a battery 1 and a load 2, and includes a DC-DC converter 3 and a bypass relay 20. For example, the load 2 is a vehicle audio instrument or a vehicle interior light. The DC-DC converter 3 includes a main relay 10, a booster circuit 11, an input interface 12, a CPU 13, and a transistor Q. The main relay 10 includes a coil Xa and a contact Ya. The contact Ya is in a normally-opened state. The bypass relay 20 includes a coil Xb and a contact Yb. The contact Yb is in a normally-closed state. An ignition signal from an ignition switch SW and a boosting request signal from a boosting request signal generator 4 are input to the CPU 13 through the input interface 12. For example, the boosting request signal generator 4 is an idling stop Electronic Control Unit (ECU).
While the vehicle runs, the ignition switch SW becomes ON (a closed state), and the H (High)-level ignition signal is input to the CPU 13. When receiving the H-level ignition signal, the CPU 13 controls the transistor Q in an off state. Therefore, the coil Xb of the bypass relay 20 is not energized, but the contact Yb of the bypass relay 20 is ON (the closed state). Accordingly, the DC voltage is supplied from the battery 1 to the load 2 through the contact Yb of the bypass relay 20. On the other hand, because the boosting request signal is not input to the CPU 13 from the boosting request signal generator 4, the CPU 13 does not drive the main relay 10, but the contact Ya of the main relay 10 is OFF (an opened state). Because the CPU 13 does not drive the booster circuit 11, the DC-DC converter 3 does not perform a boosting operation.
When the engine restarts after the vehicle stops to become the idling stop state, the L (Low)-level boosting request signal is input to the CPU 13 from the boosting request signal generator 4. When receiving the L-level boosting request signal, the CPU 13 turns on the transistor Q, energizes the coil Xa of the main relay 10, and drives the booster circuit 11. Because the coil Xb of the bypass relay 20 is energized from the battery 1 by turning on the transistor Q, the contact Yb of the bypass relay 20 becomes OFF. On the other hand, because the coil Xa of the main relay 10 is energized from the battery 1, the contact Ya of the main relay 10 becomes ON. Accordingly, the boosted DC voltage is supplied from the battery 1 to the load 2 through the contact Ya and the booster circuit 11.
The contact of the relay is roughly divided into a normally-opened contact and a normally-closed contact. The normally-opened contact is opened when the coil is not energized, and the normally-opened contact is closed when the coil is energized. On the other hand, the normally-closed contact is closed when the coil is not energized, and the normally-closed contact is opened when the coil is energized. In the power-supply device 300 in FIG. 17, the contact Yb of the bypass relay 20 is the normally-closed contact. This is because, in the case that the load 2 is the audio instrument or the vehicle interior light, it is necessary to drive the load 2 even if the ignition signal is not output while the vehicle is in the stopped state.
However, in the case that the contact Yb of the bypass relay 20 is the normally-closed contact, the coil Xb of the bypass relay 20 is energized to open the contact Yb when the transistor Q is turned on due to the breakdown of the CPU 13 or the transistor Q. Therefore, unfortunately the voltage is not supplied from the battery 1 to the load 2, and the load 2 cannot be driven.